Satellite communication systems typically have employed large aperture antennas and high power transmitters for establishing an uplink to the satellite. Recently, however, very small aperture antenna ground terminals ("VSAT"), referred to as remote ground terminals, have been developed for data transmission at low rates. In such systems, the remote ground terminals are utilized for communicating via a satellite from a remote location to a central hub station. The central hub station communicates with multiple remote ground terminals, and has a significantly larger antenna, as well as a significantly larger power output capability than any of the remote ground terminals.
The remote ground terminals are comprised of an indoor unit (IDU) and an outdoor unit (ODU). The indoor unit receives data from the user's equipment and transfers this data to the outdoor unit. The IDU and ODU are usually placed at the same location, but are physically separated and connected by a communication cable. The transmitter of the ODU functions to produce a carrier signal in accordance with an intermediate frequency signal received from the IDU and then transmits this signal to the satellite. Upon receipt by the central hub station, the modulated carrier signal is demodulated such that the data transmitted from the remote user is reproduced and processed by the central hub station.
In a low-rate VSAT system, it is very important that good transmit frequency accuracy be maintained. In a remote VSAT terminal, the reference frequency is sometimes corrected for long-term drift using the signal received from the VSAT hub (the "outroute") as a reference. The local frequency reference can be compared with the outroute symbol rate to determine what frequency adjustment is necessary at the remote terminal.
In order to generate the proper frequency at the remote, previous VSATs employed a combination of frequency synthesizers linked to a common stable reference frequency. The reference frequency is usually fixed to a standard frequency (such as 10 MHz). This reference circuit is stabilized using a combination of temperature control (a small temperature controlled housing surrounding the oscillator), temperature compensation, and comparison of outroute signals received from the central hub to those at the remote terminal.
Although most satellite communications activity takes place in the C band, there is a steady move toward the higher frequencies. Most new communications satellites operate in the Ku band having uplink frequencies in the 14-14.5 GHz range and downlink frequencies in the 11.7 to 12.2 GHz range.
In a remote Ku band VSAT terminal, the transmitter frequency must typically be generated to a carrier channel assignment with a resolution of about 10 KHz over a 500 MHz range. In addition, phase noise must be controlled to enable modulation at symbol rates as low as 32 KHz. Traditionally, when using synthesizers other than DDS synthesizers, these requirements have dictated the use of two synthesizers--a "fine step" synthesizer to perform channel assignment and a "coarse step" synthesizer to perform modulation. The two synthesizers may be co-located in the IDU or ODU equipment of the remote terminal. It is desirable to locate these circuits indoors because of the reduced temperature ranges to which the IDU is exposed, however, with traditional techniques locating both synthesizers in the IDU dictates the use of a wide-band Intra Facility Link ("IFL") cable because of the need to send a 500 MHz bandwidth between the IDU and ODU.
In another type of prior art synthesizer, the IDU contains the fine synthesizer for narrow range modulation and the ODU contains the coarse synthesizer. The disadvantages of this prior art construction are the temperature ranges to which the synthesizer in the ODU is subjected require that the ODU have expensive packaging and temperature compensation equipment to protect the synthesizer in the ODU.
The prior art frequency generator devices using two synthesizers do not provide the important advantage of fine resolution using a single synthesizer in the IDU in conjunction with the use of relatively inexpensive narrow bandwidth cable equipment between the IDU and the ODU.